Wireless LAN (WLAN) technology is very popular in the market due to its wireless advantages, high-speed access and low price comparable to wired access. WLAN technology has been replacing wired technologies, and is now applied in scenarios such as home, campus, hotel, and enterprise office. It is widely deployed in public hot spots as a wireless broadband access technology to provide public wireless broadband data access services. The following describes a WLAN system in detail by taking the WLAN system defined in IEEE 802.11 as an example.
FIG. 1 shows the basic structure of a WLAN system. The system includes: (i) stations (STAs), further including WLAN interfaces; (ii) access points (APs), equivalent to base stations in a mobile network and configured to establish communications between STAs or between STAs and relevant equipment in a wired network and between STAs and relevant equipment in a wired LAN. For example, multiple STAs access an AP to form a WLAN. An AP and a STA associated with an AP are called a basic service set (BSS). The AP may be configured with (see the AP and STA in BSS 2 shown in FIG. 1) or separated from the STA (see BSS1 shown in FIG. 1); and (iii) a distribution system (DS), configured to establish communications between different BSSs and between a BSS and a wired LAN. A logic point that forwards MAC service data units (MSDUs) between a DS and a wired LAN is called a portal.
The WLAN system is also called an extended service set (ESS).
In the WLAN system, a STA may work in a power saving (PS) mode, that is, the STA receives Beacon frames and processes the received control information only, but does not send any message to the AP. Generally, when no packet is sent, the STA may enter the PS mode (snooze status) and notify the AP to record the association ID (AID) and MAC address of the STA; when packets are sent to the STA in the PS mode, the AP may not send packets to the STA directly, but buffer the packets and notify the STA to take the packets through the Beacon frame. The STA in the PS mode intercepts the Beacon frame periodically. After detecting that a packet is buffered in the AP, the STA wakes up and sends a short Power Supply Poll (PS-Poll) frame to the AP; the AP sends the buffered data immediately or after responding to the PS-Poll frame. If all the STAs in the BSS work in the PS mode, the AP may buffer all the broadcast frames and multicast frames and send them to the STAs during the transmission of delivery traffic indication message (DTIM) frames.
A Beacon frame includes a traffic indication message (TIM) information domain, which is configured to indicate whether a STA in the PS mode has frames buffered in an AP. As shown in FIG. 2, the TIM information domain includes an Element ID, a Length, a DTIM Count, a DTIM Period, a Bitmap Control, and a Partial Virtual Bitmap. Each of the Element ID, Length, DTIM Count, DTIM Period, and Bitmap Control occupies one byte. The Partial Virtual Bitmap has variable lengths ranging from 1 to 251 bytes. The following describes components of the TIM information domain:
Length is configured to indicate the length of the TIM information domain unit.
DTIM Count is configured to indicate the number of Beacon frames to be transmitted before the next DTIM. If the DTIM Count is 0, the current TIM is a DTIM.
DTIM Period is configured to identify the number of Beacon frames between two successive DTIMs. If all the TIMs are DTIMs, the value of DTIM Period is 1. If the value of DTIM Period is 1, the DTIM Period is reserved.
The DTIM Count and DTIM Period are configured to notify the STA of the time when the broadcast/multicast frames buffered in an AP are sent and of the frequency of sending the broadcast and multicast frames.
Bitmap Control is configured to indicate whether broadcast/multicast frames are buffered in an AP by using the least significant bit. When one or more broadcast/multicast frames or multiple target frames are buffered in the AP, the least significant bit of the Bitmap Control domain is set to 1; when the DTIM Count domain is set to 0, the broadcast/multicast frames are sent. The remaining seven bits of the Bitmap Control domain are used as bit mapping offset values.
Partial Virtual Bitmap is configured to indicate whether the broadcast/multicast frames buffered in an AP are sent completely. The Partial Virtual Bitmap may be 251 bytes at most. When each bit associates a STA, the Partial Virtual Bitmap may associate 2008 STAs. The zero bit of the Partial Virtual Bitmap is reserved by the AP. When any of the other bits of the Partial Virtual Bitmap is set to 1, the STA associating the bit has unicast data buffered in the AP.
As shown in FIG. 2, in the TIM information domain of the Beacon frame, only the least significant bit of the Bitmap Control domain indicates whether there are broadcast/multicast frames. Thus, the Bitmap Control domain cannot indicate which multicast group the broadcast/multicast frames come from. Besides, the Bitmap Control domain cannot indicate which service network the broadcast/multicast frames come from. Thus, when the AP is shared, all the STAs associated with the AP are wakened to receive the broadcast/multicast frames, no matter whether the broadcast/multicast frames belong to the STA. The STA is connected to a service network through an associated service set ID (SSID). The service network includes an external network and/or a local network, where the external network may be a subscribed service provider network or another external network. For the convenience of description, the SSID mentioned below refers to the corresponding service network.
When a virtual AP scheme is adopted to share the AP, that is, a physical AP is simulated into multiple virtual APs, and each virtual AP associates different basic SSIDs (BSSIDs) and has independent MAC management frames. STAs with different SSIDs may be associated with different virtual APs of a same physical AP.
In the virtual AP scheme, a BSSID associates an SSID correspondingly, and it may wake up the STA associated with the virtual AP, by sending a Beacon frame from each of the virtual AP. This, however, may increase the load of radio channels. Besides, when the SSID has multiple multicast sources, this method cannot specify the multicast source, that is, all the STAs that are associated with the SSID have to be wakened to receive multicast frames.
When multiple SSIDs are adopted to share the AP, different services or user groups may select different SSIDs. In this case, the AP must be configured with multiple SSIDs, each of which associates a user group or a service. Though the AP may differentiate a user group or a service according to the SSID, the AP cannot enable the STA that is associated with the SSID to determine whether there are broadcast/multicast frames buffered in the AP.